Beamforming methods are generally used in the downlink on the part of a base station to achieve improvements in reception on mobile radio communication terminals. Improvements in the reception of transmitted signals are achieved by beamforming methods which are employed on the transmission side and which can be implemented with the aid of what is termed a SMART antenna array.
In the related art, groups of radio communication terminals are formed based on a respective angular position, it then being possible to provide radio coverage to an individual group by a directed radiation pattern.
Depending on the resolution or, as the case may be, width of the radiation patterns used to provide radio coverage to groups it is possible, because of the spatial orientation of the radiation patterns, to provide radio coverage to a comparatively high number of radio communication terminals using the same radio transmission resources (such as carrier frequency, timeslot, code, etc.). Based on the resolution, width and spatial orientation of the radiation patterns it is possible, despite the use of the same radio transmission resources, to reduce disruptive interference signals in the radio communication system as a result of the directed transmission.
For the improved use of beamforming methods it is necessary always to know the current angular positions of receiving radio stations that are to be provided with radio coverage by a transmitting radio station.
In particular in the case of a mobile radio system in which positions of mobile terminals can change quickly it is necessary to detect changes in position quickly in order, where appropriate, to be able to update a covering radiation pattern or in order to carry out a new assignment of a terminal to a radiation pattern. In this way it is made possible to avoid an existing radio connection between a transmitting radio station, for example a base station, and a receiving radio station, for example a terminal, being lost.
It is common practice at the present time to use what are termed “opportunistic beamforming” methods with which, for example, a radio cell or a predefined angular range is sampled and provided with radio coverage on the part of a base station that has a suitable antenna array with the aid of a rotatable radiation pattern.
FIG. 1 shows such an angular range WB which is embodied, for example, as a 120° sector of a radio cell. A rotatable radiation pattern SD1 is formed—generally using digital signal processing in the baseband on the part of a base station BTS1—with the aid of an adaptive beamforming method. The radiation pattern SD1 samples by way of example the illustrated angular range WB from 60° through 0° to 300°.
At measuring times a radio station FS1 determines quality values which allow a time-related assessment of a radiation pattern signal transmission. Measured “signal-to-interference-noise ratio” (SINR) values, for example, can be used as quality values.
If the radio station FS1 is detected by the rotating radiation pattern SD1, the radio station FS1 will measure a correspondingly greater SINR value than at times at which the radio station FS1 is not detected by the rotating radiation pattern SD1.
The measured SINR values are reported back to the base station BTS1. The latter decides, based on the reported SINR values, whether and how the base station BTS1 will perform a radio or, as the case may be, data transmission to the radio station FS1.
If a correspondingly large SINR value is reported back, a radio transmission of data can take place from the base station BTS1 to the radio station FS1, since the latter is detected by the rotating radiation pattern SD1.
If, on the other hand, the radio station FS1 is not detected by the rotating radiation pattern SD1, the reported SINR value is correspondingly small and no radio transmission of data would take place from the base station BTS1 to the radio station FS1.
In a method of this kind two opposing ancillary conditions need to be taken into account during the sampling in a compromise to be chosen:
On the one hand the sampling of the radio cell should be performed as quickly as possible in order to provide each radio station with data at regular intervals. Too slow a rotation of the radiation pattern could lead for example to time gaps in the radio coverage of the radio station that are undesirable in terms of a “Quality of Service” (QoS) value that is to be complied with.
On the other hand each radio station requires a certain measuring time for determining the quality value and for reporting back the quality value to the radio station that is performing the rotation of the sampling radiation pattern. In addition processing time is required there in order to evaluate the reported quality values. These required times represent an obstacle in terms of increasing the speed of rotation of the radiation pattern.
In the event that a rotation is too fast, reported SINR values, based on which the radio transmission is specified, could already be out of date and could disadvantageously influence an efficient data transmission.